Method of removing binder from powder molded products

ABSTRACT

A method of vaporizing and removing a binder from a powder-molded product containing the binder, which method comprises: 
     coating the greater part of the surface of said powder-molded product with a thin resin film having air-tightness thereby leasing an exposed surface portion; 
     pressurizing the thus-coated surface portion of the pressure-molded product hydrostatically; 
     vaporizing the binder in the power-molded product under said hydrostatic pressurization; and 
     removing the vaporized binder to the exterior of the power-molded product through said exposed surface portion not coated with the thin film.

FIELD OF ART

The present invention relates to a method of removing a binder frompowder molded products and more particularly to a method of removing abinder used as a molding assistant from powder molded products producedaccording to injection molding or slip casting to obtain sinteredceramic products, thus relating to a so-called powder molded productsdewaxing method.

BACKGROUND ART

Sintered ceramic products as mass-produced products complicated in shapeare industrially produced by molding such powders as alumina, zirconia,silicon carbide and silicon nitride as raw materials into desired shapesaccording to injection molding or slip casting, followed by dewaxing,and then igniting the thus-obtained powder molded products at atemperature required for sintering.

The injection molding referred to above is a molding method in which abinder exhibiting plasticity as a whole and making molding easier suchas, for example, polystyrene, polyethylene, diethylene phthalate,paraffin, fatty acid ester, or polyvinyl alcohol, is added to andkneaded with, for example, alumina powder as mentioned above in anamount of 20 to 35 parts by weight based on 100 parts by weight of thepowder, and the kneaded mixture is charged under pressure into a desiredshape of a mold and molded. The resulting powder-molded product is takenout of the mold and the binder is vaporized and removed by heating,followed by igniting to obtain a sintered ceramic product of a desiredshape.

The slip casting referred to above is a casting method in which 20 to 40parts by weight of a binder which is water or a mixture of water with analcohol, as well as a small amount of a peptizer such as HCl, AlCl₃,NaOH or water glass, are added to and thoroughly mixed with 100 parts byweight of, for example, alumina powder as mentioned above to obtain astable slip having fluidity and difficult for the powder to precipitate,then this slip is poured into a mold of a porous material such asgypsum, allowing at least the binder contained in the slip to beabsorbed into the mold until the slip has no longer fluidity, and thenthe resulting powder-molded product is taken out from the mold. Thepowder-molded product thus obtained still contains the binder, e.g.water, usually in an amount of 10% to 15% by weight. Therefore, as inthe case of injection molding, such remaining binder is vaporized andremoved by heating, followed by igniting to a temperature of, say,1,300°-2,300° C., whereby a sintered ceramic product can be obtained.

A thermoplasticizer, a plasticizer, a dispersant and a solvent added toa powder in the injection molding and slip casting in the presentinvention will hereinafter be named generically as "binder". And theoperation for vaporizing and removing, by heating or any other suitablemeans, the binder remaining in a molded product obtained according tothe foregoing injection molding or slip casting, will hereinafter bereferred to as "dewaxing" which term is commonly used by those skilledin art.

However, sintered ceramic products resulting from dewaxing andsubsequent igniting of powder molded products obtained by the foregoingmethod, i.e., injection molding or slip casting, involve the problemthat they are often defective (incapable of being used as products) dueto cracking or delamination.

Further, once such defects occur in the interior of the sintered ceramicproducts, it is difficult to find out the defects at the stage ofcommercialization, so parts of such defective products arecommercialized as they are, thus causing breakage in use. This is aserious problem.

In this connection, it is to be specially noted that such defects ascracking and delamination occur in the dewaxing step in most cases.

More particularly, if a binder remains in a powder-molded product, theremaining binder will vaporize rapidly when igniting the powder-moldedproduct into a sintered ceramic product, thus causing fracture orcracking in the same product. To prevent this, that is, to remove thebinder, the powder-molded product is subjected to dewaxing before theignition. Inevitably, therefore, it is desirable to remove the binder ascompletely as possible in the dewaxing step.

As previously noted, however, powder molded products contain not lessthan 10 wt. % of a binder even in the case of slip casting and a largeramount, not less than 20 wt. %, of a binder in the case of injectionmolding. It is essentially extremely difficult to vaporize and removethe binder by heating from the powder molded products containing thebinder in such a large amount without causing fracture or crackingbecause a large expansion force of the binder induced by the heatvaporization is exerted strongly on the powder molded products whosemechanical strength is very low.

Therefore, this process has heretofore been carried out by heating thepowder molded products to a temperature of 600° C. or so at the highestat atmospheric pressure or under a pressure of 5 kg/cm² or lower toremove the binder through evaporation, efflux or combustion. In order tokeep low the expansion force of the binder, there is adopted anextremely low heat-up rate for the powder molded product which rangesfrom 1° to 3° C./h. The dewaxing step usually requires a long period of5 to 7 days because it cannot help adopting such low heat-up rate, thusimpeding the productivity markedly.

In injection molding, moreover, a large amount of the binder used wouldresult in increased volume of voids formed after removal of the binderin the dewaxing step. Therefore, it is required that the amount of thebinder used be as small as possible and that the binder be easily moldedand have properties capable of being easily heat-vaporized. However,there has been the problem that it is essentially extremely difficult tosatisfy all of the above requirements even in the use of such expensivematerials as the foregoing polystyrene and polyethylene.

Further since the powder molded products after dewaxing are almost zeroin mechanical strength, fracture or cracking is apt to occur in thecourse of transfer to the next sintering step. In order to avoid this,it has been necessary to minimize vibration and deflection.

Thus, although the productivity is extremely low and the production isperformed through extremely careful operations, not a few sinteredceramic products obtained are defective. This has mainly beenattributable to the defects of powder molded products occurring in thedewaxing step.

Having made extensive studies for thoroughly eliminating the drawbacksof the prior art, the present inventors found out a method capable ofsuppressing the occurrence of such defects as fracture and cracking to aremarkable extent as compared with that in the prior art even when thebinder was removed in an extremely short time. In this way the presentinvention was completed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are perspective views showing examples of shapes of powdermolded products used in the present invention; and

FIGS. 3 and 4 are sectional views of powder molded products used in thepresent invention as attached to a pressure vessel.

DISCLOSURE OF THE INVENTION

According to the present invention there is provided a method ofvaporizing and removing a binder from a powder-molded product containingthe binder, which method comprises coating the greater part of thesurface of said powder-molded product with a thin resin film havingair-tightness thereby leaving an exposed surface portion, pressurizingthe coated surface of the molded product hydrostatically, allowing thebinder contained in the molded product to vaporize under the hydrostaticpressure, and allowing the vaporized binder to escape to the exterior ofthe molded product through said exposed surface portion.

In the present invention, in dewaxing a powder-molded product obtainedby, for example, injection molding or slip casting and still containinga binder, the surface of the powder-molded product exposed is coatedwith a thin resin film having airtightness while remaining a part of thesurface uncoated.

Such thin film can be formed on the surface by, for example, applying aliquid resin which solidifies upon evaporation of a solvent or achemical reaction thinly onto the surface of the molded product directlyor by spraying or by dipping and pulling up, followed by drying orheating as necessary. Preferably, the resin used for this purpose canafford a thin film having airtightness, namely, gas impermeability andalso having elasticity and/or flexibility so that a hydrostaticallypressurizing action can be exerted effectively on the molded product. Asexamples of the said resin are mentioned such industrially manufacturedresins as epoxy resins, acrylic resins, polyester resins, chloropreneresins, polyurethane resins, silicone resins, vinyl acetate resins,styrene-butadiene rubber, acrylic rubbers, natural rubbers, and phenolicresins. The liquid resin may take a suitable form according to the kindof resin used; for example, it may be in the form of latex, emulsion orsolution. Moreover, among the acrylic, epoxy and polyester resins thereare included those which when applied in a powdery state and heated arefused to form a coating. Such resins are also employable.

The thickness of the thin film may be decided suitably in view of theshape of the powder-molded product, particle size of the powder,pressure in the hydrostatic pressurization, and the kind of the thinfilm, provided it should be not smaller than the minimum thicknessrequired for maintaining airtightness.

According to experimental knowledge of the present inventors, it isdesirable that the thickness of the thin film be usually not smallerthan 10 μm. There is no special upper limit of the thin film thickness,but for convenience' sake in handling, it is desirable that the thinfilm thickness be up to 5 mm or so. Of course, even larger values areemployable depending on the kind of the thin film.

The thin film constituting resin preferably has a certain degree ofelasticity (including flexibility) as previously noted. This is becausethe thin film formed of the resin exhibits the function that hydrostaticpressure is transmitted effectively to the molded product through thethin film which is in close contact with the surface of the moldedproduct and the voids formed in the molded product as a result ofvaporization of the binder are eliminated effectively by isotropiccontraction caused by the hydrostatic pressurization. The degree ofelasticity of the resin is not specially limited as long as the thinfilm exhibits such function effectively. For example, as a measure ofselection, the glass transition point of the thin film is not higherthan the dewaxing temperature.

In the present invention it is necessary that a part of the surface ofthe powder-molded product be exposed without coating, thereby allowingthe vaporized binder to escape from the thus-exposed surface portion.The position of the exposed surface portion should be decided inconsideration of the shape of the powder-molded product and a partialmechanical load applied to a sintered ceramic product obtained from thepowder-molded product. For example, if the shape of the powder-moldedproduct is symmetric with respect to an axial, it is preferable that theexposed surface portion be located at an axial end because of easinessof the pressurizing operation. Further, it is preferable that if thepowder-molded product is in the form of a cylinder, the exposed surfaceportion be one end section, as shown in FIG. 1 and that if it is in theshape of a propeller, the exposed surface portion be one end section ofthe rotating shaft. The reason is that in the vicinity of the exposedsurface portion the powder is difficult to be pressurized uniformly, sothe mechanical load is not large in the form of a sintered ceramicproduct. Such exposed surface area portion as shown in FIGS. 1 and 2satisfies the conditions mentioned above.

From the exposed surface portion there escapes the vaporized binder, soif the area of this surface is too small, a longer time will be requiredfor dewaxing, while a too large area of this surface would result inincrease in the proportion of the portion of the powder-molded productwhich portion is difficult to be compressed uniformly. An appropriatearea of the exposed surface portion is decided in consideration of theabove tendency as well as the size and shape of the powder-moldedproduct. According to the present inventors' knowledge obtained throughexperiments, the exposed area is in the range of 0.5 to 20%, preferably1 to 10%, of the total surface area. Usually, one such exposed surfaceportion may be provided, but a plurality of such exposed surfaceportions may be provided depending on the shape of the powder-moldedproduct, etc.

In this way a portion of the powder-molded product is exposed and theremaining portion is coated with the thin film. The thus-coated surfaceportion is first subjected to hydrostatic pressurization.

The hydrostatic pressurization may be carried out by dipping the coatedsurface in liquid and pressurizing the liquid with a pump or the like. Asuitable example of the liquid used for this purpose is about 30 wt. %of aqueous boric acid or hydraulic oil. The pressure to be applied tothe liquid should be decided so that the expansion force induced uponheating of the binder does not cause defects such as cracking in thepowder-molded product. It is suitably selected according to the kind ofbinder, heating temperature and the shape of powder-molded product. Forattaining the purpose in question, the said pressure is preferably notlower than 5 kg/cm².

Further, it is preferable that the hydrostatic pressure be set at alevel at which the voids formed by vaporization of the binder can beeliminated by isotropic contraction of the powder-molded product uponhydrostatic pressurization. In view of this point it is desirable thatthe hydrostatic pressure be in the range of 500 kg/cm² or more to 10tons/cm² or less.

For pressurizing only the coated surface, there may be adopted, forexample, such a method as illustrated in FIG. 3. As shown therein, apowder-molded product 3 and a hollow pressure-resisting pipe 5 areinterconnected and in this state the surface of the powder-moldedproduct and the outer surface of the pressure-resisting pipe are coatedwith the same thin film. As a result, the surface of the powder-moldedproduct in contact with the interior of the hollow pressure-resistingpipe is an exposed surface not coated with the thin film. In this statethe coated surface may be pressurized.

It is preferable that the exposed surface be compressed by a suitablemethod so as not to induce distortion in the vicinity thereof. Forexample, it is effective to bring a gas permeable porous pipe intocontact with the exposed surface within the hollow pressure-resistingpipe, as shown in FIG. 4. The porous pipe has a pore diameter not largerthan 5 mm, preferably not larger than 1 mm, more preferably not largerthan 0.1 mm and not smaller than 0.01 μm.

In such a hydrostatically pressurized state of the coated surface of thepowder-molded product, the binder contained therein is vaporized. Thevaporization is effected by heating the powder-molded product from theexterior to vaporize or decompose the binder and/or by vacuum suctionfor the powder-molded product through the exposed surface.

The thus-vaporized binder is allowed to escape and removed to theexterior through the exposed surface portion of the molded product notcoated with the thin film.

The heating for the powder-molded product to vaporize the binder as amolding aid is done through pressurized liquid such as pressurizedaqueous boric acid or hydraulic oil. Heat-up rate, ultimate temperatureand holding time are suitably selected according to the kind of binder.

Reference will now be made to a preferred mode of vaporizing andremoving the binder by heating. In the range from room temperature up toa temperature lower by about 15° C. than the boiling point of a volatilecomponent contained in the binder, the heating may be done at anydesired rate, but once such temperature is reached, the powder-moldedproduct is held in the temperature range of from such temperature to atemperature lower by about 2° C. than the boiling point of the volatilecomponent, for 3 to 10 hours, allowing approximately 40-60% of thevolatile component to be vaporized and removed during that period. Then,heating is made to a temperature not lower than the boiling point of thevolatile component to vaporize and remove the remaining volatilecomponent. The dewaxing operation is now completed.

In heating the powder-molded product in the above manner, suction ma bemade through the exposed surface portion by means of a blower. This isalso a preferred made for vaporizing the volatile component rapidly.

Further, the mode of vaporizing and removing the binder by vacuumsuction-deaeration through the exposed surface portion can be effectedby maintaining a spatial portion 7 which is in contact with the exposedsurface portion as shown in FIG. 3 at a reduced pressure by means of avacuum generator (not shown) such as a vacuum pump or an ejector.

In this way the binder is subjected to vacuum suction-deaeration andallowed to escape to the exterior through the exposed surface portion,and thus the powder-molded product is dewaxed easily. The degree of thevacuum reduction can be varied also according to the kind of the binderto be dewaxed. But, usually it is not higher than 700, preferably 500,more preferably 100 and still more preferably 10, mmHg abs., and notlower than 10⁻⁵ mmHg abs. In performing the vacuum suction-deaeration,it is more desirable that not only the spatial portion in contact withthe exposed surface portion but also the interior of the powder-moldedproduct be held at a reduced pressure using a vacuum generator of alarge capacity.

In the case of removing the binder by vacuum suction-deaeration, it isnot always necessary to heat the powder-molded product in the dewaxingstep. But when heating is made, there may be adopted a temperature muchlower than that in the conventional process. That is, the degree ofpressure reduction and the temperature should be suitably selected inconsideration of the boiling point (or vapor pressure-temperature curve)of the material to be dewaxed. It goes without saying that the higherthe degree of pressure reduction, the lower can be the (heating)temperature of the powder-molded product. Even room temperature may beadopted in some case.

And as previously noted, the heating is effected through pressurizedaqueous boric acid or hydraulic oil.

The present invention has the following advantageous effects. Accordingto the method of the present invention, a binder contained in apowder-molded product is vaporized and removed while pressurizing thepowder-molded product hydrostatically, so even when voids are formed inthe interior of the molded product, such voids can be eliminated easilyby isotropic contraction of the molded product induced by thehydrostatic pressurization. Consequently, where injection molding isadopted in the present invention, it is not always necessary to use asthe binder such expensive material as has heretofore been used, e.g.polystyrene or polyethylene. There may be used even a viscous 0.1-5%solution of a water-soluble high polymer such as polyvinyl alcohol,carboxymethyl cellulose or polyethylene glycol in water, or a 1-20%solution of an oil such as lauric acid, palmitic acid, stearic acid orglycerin in alcohol. These solution are inexpensive.

Where slip casting is adopted in the present invention, there is usedthe same binder as in the prior art such as, for example, a bindercomprising water or a mixture of water and an alcohol and a small amountof a peptizer such as HCl, AlCl₃, NaOH or water glass.

Since the above materials lower in boiling point than those usedheretofore are also employable as binder in the present invention, theheating in the dewaxing step can be done at a lower temperature.

INDUSTRIAL UTILIZABILITY

In the method of the present invention, a part of the surface of thepowder-molded product is exposed, while the remaining surface portion iscoated with a thin film having both airtightness and elasticity, thenthe binder contained therein is vaporized and removed through the aboveexposed surface portion under application of a hydrostatic pressure of,say, 500 kg/cm² or higher. According to the present invention,therefore, the expansion force induced by the vaporization of the bindercan be suppressed effectively by the said hydrostatic pressure.Consequently, the dewaxing operation can be carried out in an extremelyshort time and the occurrence of defects such as fracture and crackingcaused by the dewaxing can be suppressed to a greater extent than in theprior art.

The dewaxing operation in the present invention is performed in theabove manner, so when the volume of the powder-molded product is, say, 1l or so, the dewaxing operation can be completed within 24 hours, farshorter than in the prior art.

Moreover, since the voids formed in the interior of the powder-moldedproduct after the removal of the binder can be eliminated easily byisotropic contraction of the molded product, it is not always requiredto use the expensive polystyrene which has been developed as a bindereasily exhibiting plasticity when kneaded with powder even in a smallamount and capable of being vaporized and removed easily by heating.

Further, since the thin film coated over the powder-molded product alsoserves to reinforce the same product, it is possible to prevent fractureand cracking which may occur in the course of transfer to the sinteringstep of the powder-molded product after dewaxing.

Additionally, since lower boiling materials than polystyrene andpolyethylene are employable as binders in the present invention, it isnot necessary to perform the dewaxing operation at such a hightemperature as 600° C. or so which is required in the use of polystyreneor polyethylene. Some such materials permit execution of the dewaxingoperation at a far lower temperature, for example, at a temperature inthe range of room temperature to 250° C. Thus, the present invention isextremely advantageous also in point of heat energy consumed.

Since the present invention has such advantageous effects, it can beeffectively applied particularly to the production of sintered ceramicproducts which are relatively complicated in shape requiring injectionmolding or slip casting and which are required to have reliability inmechanical strength.

EXAMPLE 1

As starting powders there were used 100 parts by weight of siliconnitride powder having a specific surface area of 15 m² /g and an averageparticle diameter of 0.3 μ as measured using an electron microscopeimage and 5 parts by weight of magnesium oxide having a specific surfacearea of 12 m² /g and an average particle diameter of 0.35 μ as measuredusing an electron microscope image. 25 parts by weight of ethanol and 5parts by weight of lauric acid were added as a binder to the startingpowders, followed by kneading. The resultant mixture was subjected toinjection molding at an injection pressure of 500 kg/cm² to obtain acylindrical, 15 mm dia. by 50 mm long, powder-molded product containingsilicon nitride. The proportion by volume (hereinafter referred to as"percent powder packing") of the starting powders relative to theapparent volume of the powder-molded product obtained was 57%.

As shown in FIG. 4, a porous body 8 (diameter: 15 mm, length: 10 mm,average pore diameter: 10 μ) made of alumina was put on one end portionof the powder-molded product indicated by the numeral 3 and connected toa hollow pressure-resisting pipe 5 fixed to a pressure-resisting vessel.In this state the surfaces of the molded product, porous body and pipewere coated with a thin film 2 having a thickness of 120 μm. The percentarea of the exposed surface portion was 6%. The coating was performed byapplying a liquid styrene-butadiene latex (a copolymer of 60% styreneand 40% butadiene) to the powder-molded product according to adip-pulling up method and then drying off the water contained in thelatex to form a thin film on the surface.

Next, the dewaxing step was carried out by filling thepressure-resisting vessel with 30 wt. % of aqueous boric acid andheating the aqueous boric acid with a heater in a pressurized state to1500 kg/cm² using a pump to raise the temperature of the powder-moldedproduct. The heating using the aqueous boric acid was done in thefollowing manner.

The temperature of the powder-molded product was raised from roomtemperature up to 75° C. at a rate of 30° C./h, and after holding at 75°C. for 3 hours, the temperature was raised from 75° C. to 174° C. at arate of 30° C./h, and after holding at 174° C. for 2 hours, thetemperature was raised to 210° C. at a rate of 30° C./h, and afterholding at 210° C. for 1 hour, the molded product was allowed to cooldown to room temperature. The total time from the start of heating tothe cooling to room temperature was 19 hours. During this period theaqueous boric acid was held in a pressurized state of 1500 kg/cm².

The ethanol and lauric acid contained in powder-molded product werevaporized and removed from the exposed surface portion no coated withthe thin film to the exterior of the pressure-resisting vessel throughthe porous body 8 made of alumina and the hollow pressure-resisting pipe5. The pressure of the said exterior was atmospheric pressure.

The powder-molded product 3 taken out from the pressure-resisting vesselwas free from any such change in appearance as cracking or breakage ofthe thin film. The ethanol and lauric acid were vaporized and removednot less than 99%. The percent powder packing was 62%, showing anincrease as compared with that before dewaxing.

The powder-molded product was then heated at 1,800° C. for 2 hours in anitrogen gas atmosphere of 5 kg/cm² to obtain a sintered ceramicproduct. The density of the sintered product was 3.14 g/cm³,corresponding to 99% of the theoretical density of silicon nitride.

Twenty test pieces were cut out from the sintered ceramic product andmeasured for bending strength in accordance with JIS R-1601. As aresult, an average strength and a standard deviation were 82 kg/mm² and3.1 kg/mm², respectively.

COMPARATIVE EXAMPLE 1

A cylindrical powder-molded product containing silicon nitride, preparedby injection molding in just the same way as in Example 1 washeat-dewaxed according to just the same heating method as in Example 1,in air of atmospheric pressure, directly without thin coating.

In the powder-molded product after dewaxing were found a number ofcracks at intervals of 2 to 4 mm, and exfoliations 1 to 2 mm thick wereobserved over approximately 40% of the surface.

COMPARATIVE EXAMPLE 2

The same silicon nitride powder and magnesium oxide as in Example 1 wereused in amounts of 100 and 5 parts by weight, respectively, as startingpowders, to which were added polypropylene, polyethylene and stearicacid in amounts of 19, 10 and 1 parts by weight, respectively, followedby kneading. The mixture thus obtained was subjected to injectionmolding in just the same manner as in Example 1 to obtain apowder-molded product having a percent powder packing of 59%.

The powder-molded product was then dewaxed by the following conventionalheating method. The temperature of the powder-molded product was raisedfrom room temperature up to 100° C. at a rate of 30° C./h, then from100° C. to 600° C. at a rate of 2° C./h, and after holding at 600° C.for 2 hours, the molded product was allowed to cool down to roomtemperature. A nitrogen gas atmosphere was adopted except in the stageof holding at 600° C. for 2 hours in which stage was adopted an airatmosphere for oxidative destruction of the binder. And the pressure wasatmospheric pressure. The total time from the start of heating to thecooling to room temperature was 260 hours. In the powder-molded producttaken out from the vessel was found no such change in appearance ascracking or delamination. Not less than 99.5% of the binder, includingpolypropylene, was vaporized and removed. The percent powder packing wasthe same as that before dewaxing, i.e. 59%.

The powder-molded product was then heated at 1,800° C. for 2 hours injust the same manner as in Example 1 to afford a sintered product havinga density of 3.11 g/cm³.

Test pieces were cut out from the sintered product and measured forbending strength in just the same manner as in Example 1. As a result,an average strength and a standard deviation were 68 kg/mm² and 6.6kg/mm², respectively.

A comparison between Example 1 and Comparative Example 1 shows that itis extremely effective for the prevention of cracking and delaminationto coat the powder-molded product with a thin film, pressurize thecoated surface hydrostatically and dewax the molded product in thispressurized condition.

From a comparison between Example 1 and Comparative Example 2 relatingto a conventional dewaxing method it is seen that the percent powderpacking of the powder-molded product after dewaxing according to thepresent invention is higher than that of the prior art and that thebending strength of the sintered product obtained according to thepresent invention is large and variations thereof are much smaller.

EXAMPLES 2-4

The same silicon nitride powder and magnesium oxide powder as in Example1 were used as starting powders in amounts of 100 and 5 parts by weight,respectively. To the starting powders were added the binders shown inTable 1 in the respective amounts (part by weight) described in the sametable, followed by kneading. The mixtures thus obtained were subjectedto injection molding in just the same way as in Example 1 to affordpowder-molded products containing silicon nitride and having the valuesof percent powder packing shown in Table 1.

Then, in the same manner as in Example 1 the powder-molded products thusobtained were each connected to a hollow pressure-resisting pipe througha porous body made of alumina as shown in FIG. 4 and in this state thesurfaces of the molded product, porous body and pipe were coated with athin film. The coating was effected by the application of an acrylicemulsion and subsequent removal of water by drying in Example 2, and bythe application of chloroprene resin in ethyl acetate as solvent andsubsequent removal of the solvent by drying in Examples 3 and 4. Thethin films were 120 μm thick in Example 2 and 230 μm in Examples 3 and4.

The thus-coated surfaces of the powder-molded products were then raisedin its temperature and dewaxed by heating 30 wt. % aqueous boric acidwith a heater under the application of a hydrostatic pressure of 1,500kg/cm² in the same manner as in Example 1.

In Example 2 the heating was done in just the same way as in Example 1,while in Examples 3 and 4 the heating was effected in the followingmanner. The temperature of each powder-molded product was raised fromroom temperature to 98° C. at a rate of 30° C./h, and after holding at98° C. for 5 hours, from 98° C. to 110° C. at a rate of 10° C./h, andafter holding at 110° C. for 2 hours, the molded products were allowedto cool down to room temperature. The total time from the start of theheating to the cooling to room temperature was 15 hours.

In the powder-molded products taken out from the pressure-resistingvessel there was found no such change in appearance as cracking orbreakage of the thin film. In all of them, not less than 99% of eachbinder was vaporized and removed. The values of percent powder packingwere all larger than those before dewaxing, as shown in Table 1.

Then, the powder-molded products were heated at 1,800° C. for 2 hours injust the same way as in Example 1 to obtain sintered products having thedensities described in Table 1.

Test pieces were cut out from those sintered products in just the samemanner as in Example 1 and measured for bending strength. Average valuesand standard deviations are as shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Example          2          3       4                                         ______________________________________                                        Binder Kind          EtOH       Water Water                                          Part by weight                                                                              25         30    35                                             Kind          Lauric Acid                                                                              PVA*  CMC**                                          Part by weight                                                                              5          0.5   0.5                                     Percent Powder Packing                                                                         57         58      56                                        (before dewaxing) %                                                           Percent Powder Packing                                                                         62         62      61                                        (after dewaxing) %                                                            Density of Sintered Product                                                                    3.14       3.15    3.13                                      g/cm.sup.3                                                                    Bending                                                                              Average Value 81         84    75                                      Strength                                                                             kg/mm.sup.2                                                                   Standard Deviation                                                                          3.6        2.5   3.9                                            kg/mm.sup.2                                                            ______________________________________                                         *Polyvinyl alcohol                                                            **Carboxymethyl cellulose                                                

EXAMPLES 5-7

The following experiment was made for the purpose of confirming theeffect of hydrostatic pressures. As starting powders there were usedsilicon carbide powder having a specific surface area of 17 m² /g and anaverage particle diameter of 0.25 μ as measured using an electronmicroscope image, elemental boron having a specific surface area of 10m² /g and an average particle diameter of 0.4 μ, and carbon black havinga specific surface area of 90 m² /g and an average particle diameter of0.03 μ, in amounts of 100, 1 and 2 parts by weight, respectively. Tothose starting powders were added as a binder 30 parts by weight ofisopropanol and 3 parts by weight of myristic acid, followed bykneading. The resulting mixture was subjected to injection molding injust the same way as in Example 1. In this way there were obtained threepowder-molded products each having a percent powder packing of 57%.

Then, in just the same manner as in Example 1 the powder-molded productsthus obtained were each connected to the hollow pressure-resisting pipe5 through the porous body 8 made of alumina and in this state thesurfaces of the molded product, porous body and pipe were coated withstyrene-butadiene latex, followed by drying to remove water to form athin film of 120 μ thickness on those surfaces.

Then, heating was made in just the same manner as in Example 1 whilemaintaining the thus-coated surfaces of the powder-molded products in ahydrostatically pressurized state at the pressures shown in Table 2. Inthis way dewaxing was effected. The values of percent powder packingafter the dewaxing are as described in the same table.

The thus-dewaxed powder-molded products were then heated at 2,050° C. invacuum (not higher than 1 mmHg) for 1 hour to obtain sintered productscontaining silicon carbide. The densities of the sintered products areas shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Example            5         6      7                                         ______________________________________                                        Hydrostatic Pressure                                                                             10        500    2000                                      kg/cm.sup.2 · G                                                      Percent Powder Packing                                                                           57        59     62                                        (after dewaxing) %                                                            Density of Sintered Product                                                                      3.08      3.14   3.17                                      g/cm.sup.3                                                                    Bending  Average Value 51        69   75                                      Strength kg/mm.sup.2                                                                   Standard Deviation                                                                          6.3       5.2  4.1                                              kg/mm.sup.2                                                          ______________________________________                                    

EXAMPLE 8

As starting powders there were used 100 parts by weight of siliconnitride powder having a specific surface area of 15 m² /g and an averageparticle diameter of 0.3 μ as measured using an electron microscopeimage, 3 parts by weight of aluminum oxide having a specific surfacearea of 20 m² /g, and 2 parts by weight of yttrium oxide having aspecific surface area of 16 m² /g. To the starting powders was added asa binder 35 parts by weight of water containing 0.2 wt. % of polyvinylalcohol dissolved therein, followed by kneading. The resulting mixturewas subjected to injection molding at an injection pressure of 500kg/cm². G to obtain a cylindrical, 15 mm dia. by 50 mm long,powder-molded product containing silicon nitride. The percent powderpacking of the powder-molded product obtained was 57%.

Then, as shown in FIG. 4, a porous body 8 (diameter: 15 mm, length: 10mm, average pore diameter: 10 μ) made of alumina was put on one endportion of the powder-molded product indicated by the numeral 3 andconnected to a hollow pressure-resisting pipe 5 fixed to apressure-resisting vessel. In this state the surfaces of the moldedproduct, porous body and pipe were coated with a thin film 2 having athickness of 120 μ. The coating was performed by applying a liquidstyrene-butadiene latex (a copolymer of 60% styrene and 40% butadiene)to the powder-molded product and then drying off the water contained inthe latex to form a thin film on the surface.

The powder-molded product thus coated with the thin film was thensubjected to dewaxing. The dewaxing step was carried out by filling thepressure-resisting vessel with 30 wt. % of aqueous boric acid,pressurizing the aqueous boric acid to 1,500 kg/cm². G by means of apump, then reducing the pressure of the space 7 to 1˜10 mmHg abs. andmaintaining this state for 5 hours. The temperature of the aqueous boricacid was set at 60° C.

The powder-molded product 3 taken out from the pressure-resisting vesselwas free from any such change in appearance as cracking or breakage ofthe thin film. Not less than 99% of the water used as a binder wasvaporized and removed. The percent powder packing was 62%, showing anincrease as compared with that before dewaxing.

The powder-molded product was then heated at 1,800° C. for 2 hours in anitrogen gas atmosphere of 5 kg/cm². G to obtain a sintered ceramicproduct. The density of the sintered product was 3.14 g/cm³,corresponding to 99% of the theoretical density of silicon nitride.

Twenty test pieces were cut out from the sintered product and measuredfor bending strength in accordance with JIS R-1601. As a result, anaverage strength and a standard deviation were 86 kg/mm² and 3.0 kg/mm²,respectively.

COMPARATIVE EXAMPLE 3

A cylindrical powder-molded product containing silicon nitride, obtainedby injection molding in just the same way as in Example 8, was reducedin pressure to 1˜10 mmHg abs. directly without thin coating and in thisstate it was subjected to dewaxing in just the same manner as in Example8. In the powder-molded product after dewaxing were found a number ofcracks at intervals of 2 to 4 mm, and delaminations 1 to 2 mm thick wereobserved over approximately 50% of the surface.

COMPARATIVE EXAMPLE 4

The same silicon nitride powder, aluminum oxide and yttrium oxide as inExample 8 were used in amounts of 100, 3 and 2 parts by weight,respectively, as starting powders, to which were added polypropylene,polyethylene and stearic acid in amounts of 14, 10 and 1 parts byweight, respectively, followed by kneading. The mixture thus obtainedwas subjected to injection molding in just the same way as in Example 8to obtain a powder-molded product having a percent powder packing of59%.

The powder-molded product was then dewaxed by the following conventionalheating method. The temperature of the powder-molded product was raisedfrom room temperature to 100° C. at a rate of 30° C./h, then from 100°C. to 600° C. at a rate of 2° C./h, and after holding at 600° C. for 2hours, the molded product was allowed to cool down to room temperature.A nitrogen gas atmosphere was adopted except in the stage of holding at600° C. for 2 hours in which stage was adopted an air atmosphere foroxidative destruction of the binder. And the pressure was atmosphericpressure. The total time from the start of heating to the cooling toroom temperature was 260 hours.

In the powder-molded product taken out from the vessel was found no suchchange in appearance as cracking or delamination. Not less than 99.5% ofthe binder, including polypropylene, was vaporized and removed. Thepresent powder packing was the same as that before dewaxing, i.e. 59%.

The powder-molded product was then heated at 1,800° C. for 2 hours injust the same manner as in Example 8 to afford a sintered product havinga density of 3.12 g/cm³.

Test pieces were cut out from the sintered product and measured forbending strength. As a result, an average strength and a standarddeviation were 74 kg/mm² and 5.9 kg/mm², respectively.

A comparison between Example 8 and Comparative Example 3 shows that itis extremely effective for the prevention of cracking and delaminationto coat the powder-molded product with a thin film, pressurize thecoated surface hydrostatically and dewax the molded product in apressure-reduced condition of the exposed surface portion.

Further, from a comparison between Example 8 and Comparative Example 4relating to a conventional dewaxing method it is seen that the percentpowder packing of the powder-molded product after dewaxing according tothe present invention is higher than that of the prior art and that thebending strength of the sintered product obtained according to thepresent invention is large and variations thereof are much smaller.

EXAMPLES 9-11

The same silicon nitride powder, aluminum oxide and yttrium oxide as inExample 8 were used as starting powders in amounts of 100, 3 and 2 partsby weight, respectively. To the starting powders were added the bindersshown in Table 3 in the respective amounts (part by weight) described inthe same table, followed by kneading. The mixtures thus obtained weresubjected to injection molding in just the same way as in Example 8 toafford powder-molded products containing silicon nitride and having thevalues of percent powder packing shown in Table 3.

Then, in the same manner as in Example 8 the powder-molded products thusobtained were each connected to a hollow pressure-resisting pipe througha porous body made of alumina as shown in FIG. 4 and in this state thesurfaces of the molded product, porous body and pipe were coated with athin film. The coating was effected by the application of an acrylicemulsion and subsequent removal of water by drying in Example 9, and bythe application of chloroprene resin in ethyl acetate as solvent andsubsequent removal of the solvent by drying in Examples 10 and 11. Thethin films were 140 μ thick in Example 9 and 210 μ in Examples 10 and11.

Then, in the same way as in Example 8 the coated surfaces of thepressure-molded products were pressurized hydrostatically to 1,500kg/cm². G using 30 wt. % aqueous boric acid held at 60° C. and in thisstate the space 7 was pressure-reduced to 1˜10 mmHg abs. This conditionwas held for 5 hours to thereby effect dewaxing.

In the powder-molded products taken out from the pressure-resistingvessel there was found no such change in appearance as cracking orbreakage of the thin film. In all of them, not less than 99% of eachbinder was vaporized and removed. The values of percent powder packingwere all larger than those before dewaxing, as shown in Table 3.

Then, the powder-molded products were heated at 1,800° C. for 2 hours injust the same manner as in Example 8 to obtain sintered products havingthe densities described in Table 3.

Test pieces were cut out from the sintered products and measured forbending strength in just the same manner as in Example 8. Average valuesand standard deviations are as shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Example          9          10       11                                       ______________________________________                                        Binder Kind          Ethanol    Methanol                                                                             Water                                         Part by weight                                                                              25         25     35                                            Kind          Lauric Acid                                                                              Glycerin                                                                             CMC*                                          Part by weight                                                                              5          5      0.5                                    Percent Powder Packing                                                                         58         57       57                                       (before dewaxing) %                                                           Percent Powder Packing                                                                         61         62       61                                       (after dewaxing) %                                                            Density of Sintered Product                                                                    3.15       3.16     3.14                                     g/cm.sup.3                                                                    Bending                                                                              Average Value 83         79     78                                     Strength                                                                             kg/mm.sup.2                                                                   Standard Deviation                                                                          3.4        3.0    3.5                                           kg/mm.sup.2                                                            ______________________________________                                         *Carboxymethyl cellulose                                                 

EXAMPLES 12-14

The following experiment was conducted for the purpose of confirming theeffect of hydrostatic pressures. As starting powders there were usedsilicon carbide powder having a specific surface area of 17 m² /g and anaverage particle diameter of 0.25 μ as measured using an electronmicroscope image, elemental boron powder having a specific surface areaof 10 m² /g and an average particle diameter of 0.4 μ, and carbon blackhaving a specific surface area of 90 m² /g and an average particlediameter of 0.03 μ, in amounts of 100, 1 and 2 parts by weight,respectively. To the starting powders was added as a binder 35 parts byweight of water containing 0.5 wt. % of polyethylene glycol dissolvedtherein, followed by kneading. The mixture thus obtained was subjectedto injection molding in just the same way as in Example 8. In this waythere were obtained three powder-molded products each having a percentpowder packing of 56%.

Those powder-molded products were each connected to the hollowpressure-resisting pipe 5 through the porous body 8 made of alumina injust the same manner as in Example 8. In this state a bisphenol A typeepoxy resin was applied to the surfaces of the molded products, porousbody and pipe and hardened to form a thin film having a thickness of 150μ.

Then, the thus-coated surfaces of the powder-molded products werepressurized hydrostatically at the pressures shown in Table 4 and inthis state there was performed dewaxing by just the same pressurereducing and heating methods as in Example 8. The values of percentpowder packing after the dewaxing are as shown in Table 4.

The powder-molded products were then heated at 2,050° C. for 1 hour invacuum (not higher than 1 mmHg abs.) to obtain sintered productscontaining silicon carbide. The densities of the sintered products areas described in Table 4.

Then, test pieces were cut out from the sintered products and measuredfor bending strength in the same manner as in Example 8. Average valuesand standard deviations are as shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Example            12        13     14                                        ______________________________________                                        Hydrostatic Pressure                                                                             10        500    2000                                      kg/cm.sup.2 · G                                                      Percent Powder Packing                                                                           57        60     60                                        (after dewaxing) %                                                            Density of Sintered Product                                                                      3.10      3.14   3.18                                      g/cm.sup.3                                                                    Bending  Average Value 54        66   73                                      Strength kg/mm.sup.2                                                                   Standard Deviation                                                                          7.1       5.4  3.8                                              kg/mm.sup.2                                                          ______________________________________                                    

What is claimed is:
 1. A method of vaporizing and removing a binder froma powder-molded product containing a binder, which methodcomprises:coating all but 0.5% to 20% of the total surface area of saidpowder-molded product with an airtight thin resin film thereby leavingan exposed surface portion hydrostatically pressurizing the thus-coatedsurface portion of the pressure molded product; vaporizing the binder inthe powder-molded product under said hydrostatic pressurization; andremoving the vaporizing binder from the powder-molded product throughsaid exposed surface portion not coated with the thin film.
 2. A methodas set forth in claim 1 wherein said vaporization of the binder of thepowder-molded product occurs by heating the powder-molded product andremoving said vaporized binder from the powder-molded product throughsaid exposed surface portion.
 3. A method as set forth in claim 1,wherein the powder-molded product is subjected to vacuumsuction-deaeration through said exposed surface portion to vaporize andremove said binder from the powder-molded product through said exposedsurface portion.
 4. A method as set forth in claim 1, wherein thepowder-molded product is heated and at the same time subjected to vacuumsuction-deaeration through said exposed surface portion to vaporize andremove said binder from the powder-molded product through said exposedsurface portion.
 5. A method as set forth in claim 1, wherein thepowder-molded product is formed by injection molding.
 6. A method as setforth in claim 1, wherein the powder-molded product is formed by slipcasting.
 7. A method as set forth in claim 1, wherein the powder is aceramic powder.
 8. A method as set forth in any one of claims 1 to 7,wherein the powder-molded product after removal of the binder issubsequently ignited into a sintered ceramic product.
 9. A method as setforth in claim 1 wherein said exposed surface portion of saidpowdered-molded product, uncovered with said thin resin film, represents1% to 10% of the total surface area of said powder-molded product.